The tautomeric equilibria of thioanalogues of nucleic acids: spectroscopic studies of 2-thiouracils in the vapour phase and in low temperature matrices

Abstract The IR absorption spectra in the NH, OH and SH stretching regions of 2-thiouracil and its N 1 -, N 3 -, O - and S -alkyl derivatives, isolated in the vapour phase and also isolated in inert low temperature matrices, and the UV absorption spectra of those compounds measured in the vapour phase are presented and discussed. These spectra establish unambiguously the tautomeric forms of these systems which are found for isolated molecules. 2-Thiouracil as well as its N 1 - and N 3 -methylated derivatives exists in the inert matrix and in the vapour phase only in the oxo-thione tautomeric form; this same form is found for these molecules in solutions and in the solid phase. In contrast, 2-methylthiouracil isolated in inert matrices and in the vapour adopts both the 4-oxo and the 4-hydroxy tautomeric forms, with an equilibrium constant K (oxo = hydroxy) of 1.2, while it exists only as the 4-oxo form in solutions and in the solid. The 4-neopentoxy derivative of 2-thiouracil, when isolated in an inert matrix, is found to be in the mercapto form only, while the thione form is dominant in the solid phase and in solution.